The requirements for the signal quality of modulators, for example in transmitting devices, become more stringent as need for high data rates and increasing mobility grows. In modern mobile radio standards, such as Universal Mobile Telecommunications System (UMTS), Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communication (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Bluetooth Medium Data Rate, or Wireless Local Area Network (WLAN) according to 802.11a/b/g, special modulation types are required for data transmission which modulate both the phase and the amplitude of a carrier signal at the same time.
Simultaneous amplitude and phase modulation make it possible to achieve higher data transmission rates and thus better bandwidth efficiency. The mobile radio standards mentioned above envisage, for example, the use of quadrature phase shift keying (QPSK), 8-phase shift keying (8-PSK) or quadrature amplitude modulation (QAM) as modulation types for data transmission.
Depending on the selected application for the individual mobile radio standards, these high-quality modulation types are used not only for data transmission from a base station to a mobile communication appliance but also from the mobile communication appliance to the base station.
The modulation types which are used for modern mobile radio standards are particularly sensitive to possible interference or distortion which is produced by various components in a transmission path. Additionally, the much more sophisticated multi-band operation and modulation formats in advanced wireless communication systems specify highly strict and difficult requirements for out-of-band emissions and spurious emissions. For example, in UMTS systems, it can be necessary to provide a radio frequency filter like a surface acoustic wave (SAW) filtering device before or after the final power amplifier in a transmission path to meet given requirements. Such radio frequency filters use a certain amount of area on a printed circuit board (PCB) and therefore increase the cost of a transmitter device.
A modulation can be performed using a vector modulation in which data to be transmitted is provided with an in-phase component and a quadrature component which form a complex signal. In other systems, also a polar modulator can be used in which the data are coded as polar coordinates with an amplitude component and a phase component. The amplitude component is usually digital analog converted and used for an amplitude modulation of a carrier signal which comprises the phase information of the phase component. A bandwidth of amplitude and phase components is usually expanded compared to the bandwidth of in-phase and quadrature components of a vector modulation.
With the use of a polar modulator or a polar transmitter, some of the radio frequency filtering efforts can be reduced. Nevertheless, there can be a need to implement a radio frequency filter also for conventional polar modulators to fulfill the out-of-band and spurious emission requirements specified by advanced mobile radio standards.
For example, through the digital processing of the data to be transmitted, the amplitude component and the phase component, a repetition spectrum can be introduced into the digital signals. The repetition spectrum is usually filtered using analogue reconstruction filters to meet the given requirements. A frequency of the repetition spectrum depends on the clock frequency used in the digital processing.